The present invention relates to an apparatus for the intermittent aeration of sewage and is more particularly concerned with an apparatus adapted to be disposed below water level which consists of a horizontal air distributor in the form of a pipe jacket closed tightly at both ends except for an aperture for the air supply, and which, in the area of its two opposite lateral longitudinal edges includes apertures for the passage of air. The apparatus further includes an elastic air ditributing foil covering the apertures for the passage of air and the upper portion of the pipe jacket located between the apertures. The foil is provided with fine air escape openings which are opened by the air supply to the pipe jacket and closed in case of interruption of the air supply by being sealed against the pipe jacket outside the area of the air passage openings.
U.S. Pat. No. 4,060,486 to Schreiber discloses an air distributor apparatus having an oval cross section together with an air distributor foil, the oval cross section being formed by the pipe jacket having in its upper area a lesser curvature and in its lower area a greater curvature. The openings for the passage of air in the two lateral longitudinal edges of the pipe jacket are disposed in the upper less arched part of the pipe jacket. Furthmore, a raised shape is provided on both sides of the top portion of the pipe jacket which projects upward over the elliptically arched shape of the pipe jacket. Immediately beside each shape and between the raised shapes and the curvature of the upper area, there is an indentation with openings for the passage of air which extend nearly over the entire length of the pipe jacket, over which stretches the air distributor foil having pricked air exit openings. The air distributor foil is thus supported by the two projecting raised shapes and by the curvature of the upper area of the pipe jacket, the two indentations having the openings for the passage of air not being covered by the foil.
As a result, a free lateral space is developed in the area of the two indentations for the unimpeded entry of air through the openings for the passage of air which does not eliminate the hydrostatic pressure acting on the air distributor submerged in the sewage since the air distributor foil is not pressed air-tight into the indentation and onto the openings for the passage of air. Even in the case of the slightest excess pressure, about 30 cm water-column pressure as compared to the water pressure of the submerged air distributor, compressed air will enter into the two lateral spaces created by the indentations, be distributed over the entire length of the air distributor and without any increased air resistance, lift the air distributor foil from the curvature of the upper area of the pipe jacket. As a result, an interval connecting the two lateral spaces and extending over the upper area of the pipe jacket develops between the jacket and the air distributor foil and the air thereby escapes through the openings of the air distributor foil into the sewage or the sewage-activated sludge mixture with about 40 cm water-column pressure.
The consequence of this operation of the known apparatus is that the compressed air is distributed uniformly in the above mentioned interval and escapes into the sewage or sewage activated-sludge mixture surrounding the air distributor as aeration air in a uniform distribution through the air escape openings of the air distributor foil. Upon completion or interruption of the air supply to the pipe jacket, the elastic air distributor foil, as a consequence of the then prevailing excess pressure of the surrounding sewage, fits against the curvature of the upper area of the pipe jacket so that the escape of air through the finely-pricked air-escape openings of the air distributor foil is stopped and any trickling in of the sludge particles is thereby avoided.
However, the manufacture of the pipe jacket described above having less of a curvature in the upper area and a greater curvature in the lower area is expensive from the view of production engineering. Additionally, the sealing at the ends of the pipe jacket becomes difficult due to the irregular oval cross section of the pipe jacket and of the closing lid adapted to fit about the jacket.